Flexible abrasive articles include coated abrasives, lapping abrasives, and nonwoven abrasives. In the case of a coated abrasive the substrate is a backing sheet. In the case of a nonwoven abrasive the substrate is a flexible open lofty porous web. In the case of lapping abrasives, the substrate is a backing.
Coated abrasives generally comprise a flexible backing sheet upon which a binder holds and supports a coating of abrasive grains. The coated abrasive may employ a "make" coat of resinous binder material in order to secure the abrasive grains to the backing as the grains are oriented, and a "size" coat of resinous binder material which can be applied over the make coat and abrasive grains in order to firmly bond the abrasive grains to the backing. The binder material of the size coat can be the same material as the binder material of the make coat or a different material.
In the manufacture of coated abrasives, the make coat and abrasive grains are first applied to the backing, then the size coat is applied, and finally, the construction is fully cured. Generally thermally curable binders provide coated abrasives with excellent properties, e.g., heat resistance. Thermally curable binders include phenolic resins, urea-formaldehyde resins, urethane resins, melamine-formaldehyde resins, epoxy resins, and alkyd resins. The most widely used binder is a resol phenolic resin.
In recent years, there has been an increasing demand for superabrasives both in the flexible and bonded abrasive markets. Superabrasives are abrasive articles that employ abrasive grains that are superior in performance, i.e., greater than 20 times that of conventional abrasive grains in abrading difficult to grind materials such as tool steels or ceramics. Superabrasive grains are typically diamond or cubic boron nitride and these abrasive grains typically cost in excess of one thousand dollars per pound. Conventional abrasive grains include garnet, silicon carbide, silica, aluminum oxide, alumina zirconia, boron carbide, and ceramic aluminum oxide. Conventional abrasive grains are typically less than ten dollars per pound.
For bonded abrasives, if superabrasive grains are employed, the binders can be vitreous, organic, or metallic (plated or sintered). While each binder type has a specific area of application, the relative strength of the binder materials is generally from strongest to weakest 1) metallic 2) vitreous and 3) organic. As a result, optimum abrasive retention and thus performance is usually achieved with metallic binders.
It is very difficult, however, to make flexible abrasive articles capable of optimum performance using metallic or vitreous binders. This is due to the processing temperatures associated with these binders. Some conventional substrates used in manufacturing flexible abrasive articles will degrade at temperatures greater than about 200.degree. C. Additionally, the metallic and vitreous binders tend to be more rigid than organic binders. This rigidity is normally not desired in a flexible abrasive article. In order to employ superabrasive grains in a flexible abrasive article, a resinous binder, such as a phenolic resin, is employed. However, phenolic resins do not always have the necessary properties to obtain the full utilization of the superabrasive grains. Thus, it is not cost effective to use superabrasive grains and consequently, superabrasive grains are not widely used in flexible abrasive articles.
U.S. Pat. No. 3,651,012 (Holub et al.) discusses a bismaleimide binder for use as insulation, protective applications and numerous molding applications. In column 13, line 33 to 45 it mentions that the bismaleimide binder can be used in bonded abrasives.
U.S. Pat. No. 4,107,125 (Lovejoy) concerns a crosslinked aromatic polyimide resin that exhibits good strength and toughness properties. This patent mentions that this resin can be employed in a bonded abrasive article.
U.S. Pat. No. 4,142,870 (Lovejoy) discloses a bonded abrasive having a combination of two linear polyimide resins as a binder.
U.S. Pat. No. 4,575,384 (Licht et al.) discloses that polyimide binders can be employed in a coated abrasive.
U.S. Pat. No. 4,729,771, (Kunimotot et al.) involves a polyimide binder for a flexible abrasive lapping film.
However, none of these references disclose a maleimide resin as a binder for a flexible coated abrasive or a method of making such an abrasive article.
A need thus exists for a flexible coated abrasive with an improved resinous binder especially for superabrasive containing constructions. The binder should possess a high degree of strength at high temperatures and under wet conditions, a high glass transition temperature, and a high modulus.